1. Field of the Invention
The present invention relates to a network address conversion system and the method thereof, more particularly to a system using a network server for performing network address conversion and the method thereof.
2. Background of the Invention
In order to solve the problem of increasingly insufficient public IP addresses, RFC1918 defines three domains for private IP addresses, 10.0.0.0 to 10.255.255.255, 172.16.0.0 to 172.31.255.255, and 192.168.0.0 to 192.168.255.255, respectively. The private IP addresses may be used to connect computers in an enterprise in view of not needing to additionally apply for more public IP addresses. However, because a plurality of private IP addresses will be converted into the same public IP address when they connect to the external public network, the object of saving IP addresses needs to be achieved. Nevertheless, because the plurality of private IP addresses share the same public IP address, the data packets should be performed in an IP address conversion through a router between the public network and the private network within an enterprise.
A common conversion method between the private IP address and the public IP address is the Network Address Translation (NAT) defined by RFC1631. The conventional network address conversion system 10, as shown in FIG. 1, uses a NAT router 12 to convert the source address included in the IP header of the data packet from the private network 11 into the public IP address of the public network 13, and convert the source address included in the IP header of the data packet from the public network 13 into the private IP address of the private network 11. For example, a node in a private network 11 having a private IP address of 192.168.1.1 can be converted to a public IP address 202.132.2.2 through a routing table of the NAT router 12, and then communicate with a remote node in the public network 13.
For the application of transmitting high quality voice over IP (VoIP), such as the Q.931 and H.245 protocols used by the videophone, the Information of IP address in the IP header is not only included in the data packet, but also in the payload. However, the conventional NAT router 12 only handles the convertion of the private IP address in the IP header into a public IP address, and not the IP address conversion in the payload. Because the IP address in the payload has not been converted, the user of the videophone in the public network 13 cannot transmit his voice and image to the user of the videophone in the private network 11.
A conventional method for solving the above problem is to add A/P gateways 21 and 22 at both ends of the NAT router 12 in cooperation with the system service provider, as shown in FIG. 2, which is responsible for the IP address conversion in the payload. However, this method increases the cost burden of the manufacturing company. For individual users, because there are no system service providers that cooperates the A/P gateways 21 and 22, the above-mentioned solution cannot be used. Another solution is to use a specific NAT router having the function of converting the IP address in the payload, but this also results in high cost burden, which is difficult to be accepted in the market. Moreover, when using ADSL or cable modem, the public IP address dynamically changes, and is not fixed, so the difficulty of the design increases.
As described above, because the current VoIP applications in the market are concerned practical requirements of the user, effectively completing the communication and reducing the cost becomes very important issues.